In the field of roboticsa nd automatic assembly tooling, it is often necessary to provide some compliance when fittng two parts together or when engaging a tool with a complementarily shaped aperture. This need arises because of the tolerances in gripping and positioning capability of a robot arm and the dimensional tolerances of the members being positioned. The use of excessive force to engage two imperfectly aligned members can lead to damage to the members or assembly tooling. The provision of compliance allows a degree of imprecision in the positioning movements and is usually cheaper than providing additional sensors to enable finer position control.
Simple compliance, in the sense of general flexibility of mechanical connections, is not, however, desirable, as this can lead to even greater misfits than the original misalignment. For example, a misaligned peg can be tilted even further off the axis of a chamfered bore by the moment of the insertion force about the edge of the bore.
In U.S. Pat. Nos. 4,098,001, 4,155,169 and 4,276,697 so-called "remote center compliance" devices are shown for interconnecting a support structure, e.g. a robot arm, to an operator device, e.g. a gripper. The devices shown are mechanical linkages of cage-like appearance with various degress of flexibility in the links. This fiexibility and the geometry of the links produces a "center of compliance", remote from the compliance device, and located nearer to the anticipated point of contact of the parts to be positioned. The center of compliance is such that a pure lateral force applied at the center of compliance will produce only a lateral translation without rotation and a pure turning force (moment) applied about the center of compliance will cause only rotation about the center with no translational movement. In both cases, movement is resisted by a controlled restoring force (or stiffness).
All of the devices described have, in common, three circumferentially spaced angled links, all pointing at the same remote center (not necessarily the center of compliance). These provide rotational compliance. In addition, either additional axially extending links or deformability of the structure supporting the links is provided to achieve lateral compliance as well. Such structures are relatively complex.
The devices described in U.S. Pat. Nos. 4,098,001 and 4,155,169 are also described by Samuel H. Drake in a doctoral thesis submitted to the Massachusetts Institute of Technology entitled "Using compliance in lieu of sensory feedback for automatic assembly" (No T-657, September 1977, published by The Charles Stark Draper Laboratory, Inc). In this thesis, at pages 136 and 137, some further modifications are illustrated including a device of simpler construction using only thick, angled, elastomeric elements that deform in both shear and compression/extension. Even this device, in common with those of the above referenced patents has the disadvantage that the stiffnesses of the various compliant motions are fixed by the materials used. There is no provision for varying the compliance for different tasks nor for removing it entirely when the robot is not in a registration/insertion mode of operation.
In U.S. Pat. No. 4,458,424, a remote compliance system is shown which employs four elastomeric spheres between a manipulator arm and a tool. These can be pressurized by fluid from a controllable supply to vary the center of compliance. The spheres resist longitudinal compression only, tension being resisted by an associated system of angled cables. Lateral movement is resisted by further spheres trapped between laterally spaced wall sections of the two devices.
Unrelated to the above types of compliant device, there is also known, from U.S. Pat. No. 4,078,671, a type of pickup device employing two tubular sections lined by means of a bellows. One of the tubular sections terminates alternatively, in either a vacuum head or an inflatable sock (designed for internal pickup). When not actually gripping an object, the tubular sections are loosely jointed together by the bellows and the pickup head is free to move in all vertical planes through the axis of the upper tube. This is said to make initial registration of the pickup head with an object to be picked up a wide tolerance operation. When vacuum or pressure is applied (depending on the type of head), the bellows either collapses or expands causing the object to be picked up and also causing alignment of the two tubes in a fixed relative position, thus removing any compliance. The object may then be moved to an assembly station by the pickup and assembled onto a mating part. The purpose of this device, unlike those discussed above, is to remove compliance at the point of assembly, in the interests of precision, by locking the two portions of the pickup together. In neither its locked not its free states does this device solve the problems of excessive force and damage caused by imperfect alignment.
The use of a system of jacks or bellows formed to connect a manipulator arm and tool is shown in European published patent application Ser. No. 0113145-A1. Multiple sets of jacks with different orientations are filled with oil and pressurized by means of plungers and the like to determine their degree of extension. Essentially, the arrangement provides controllable fine positioning of the tool with respect to the manipulator arm and is not a remote center compliance system.